Method and circuit for adjusting a self-refresh rate to maintain dynamic data at low supply voltages

ABSTRACT

A method and circuit for refreshing dynamic data stored in an integrated circuit are disclosed. The integrated circuit receives a supply voltage and operates in a self-refresh mode of operation to refresh the dynamic data at a refresh time that defines how often the dynamic data is refreshed during the self-refresh mode. The method includes monitoring a magnitude of the supply voltage and adjusting the refresh time as a function of the monitored magnitude of the supply voltage. The integrated circuit may be any type of integrated circuit that stores dynamic data, such as a memory device like a dynamic random access memory, DDR DRAM, SLDRAM, or RDRAM, or other type of integrated circuit such as a microprocessor.

TECHNICAL FIELD

[0001] The present invention relates generally to integrated circuits,and more specifically to refreshing dynamic data stored in an integratedcircuit, such as a dynamic random access memory (DRAM), as a supplyvoltage applied to the integrated circuit varies.

BACKGROUND OF THE INVENTION

[0002] Many battery-powered portable electronic devices, such as laptopcomputers, Portable Digital Assistants, cell phones, and the like,require memory devices that provide large storage capacity and low powerconsumption. To reduce the power consumption and thereby extend thebattery life in such devices, the devices typically operate in alow-power mode when the device is not being used. In the low-power mode,a supply voltage or voltages applied to electronic components such as amicroprocessor, associated control chips, and memory devices aretypically reduced to lower the power consumption of the components, aswill be appreciated by those skilled in the art. Although the supplyvoltages are varied to reduce power consumption in the low-power mode,data stored in the electronic components such as the memory devices mustbe retained.

[0003] Because large storage capacity is typically desired to maximizethe amount of available storage in portable devices, it is typicallydesirable to utilize dynamic random access memory (DRAM), which has arelatively large storage capacity, over other types of memories such asstatic random access memories (SRAM) and non-volatile memories such asFLASH memory. In a DRAM, the data is “dynamic” because the data storedin memory cells in the DRAM must be periodically recharged or“refreshed” to maintain the data, as will now be explained in moredetail with reference to FIG. 1. FIG. 1 illustrates a portion of aconventional DRAM memory-cell array 100 including a plurality of memorycells 102 arranged in rows and columns, one of which is shown in FIG. 1.The memory cell 102 includes an access transistor 104 and a storagecapacitor 106 connected in series between a digit line DL and areference voltage VCC/2. The storage capacitor 106 includes a firstconductive plate 107 coupled to the access transistor 104 and a secondconductive plate 109 coupled to the reference voltage VCC/2.

[0004] A word line WL activates the access transistor 104 in the memorycell 102, and also activates the access transistors of all other memorycells (not shown) contained in the same row of the array 100 as thememory cell 102. To write data into the memory cell 102, a senseamplifier 108 drives the digit line DL and a complementary digit lineDL*to complementary voltage levels corresponding to the data to bestored in the memory cell. The word line WL is then activated, turningON the access transistor 104 and transferring charge through the accesstransistor to charge the storage capacitor 106 to the voltage level onthe digit line DL corresponding to the data to be stored. The word lineWL is thereafter deactivated, turning OFF the access transistor 104 andisolating the storage capacitor 106 from the digit line DL to therebystore the data in the form of a voltage across the storage capacitor.

[0005] To read data from the memory cell 102, the sense amplifier 108equilibrates the digit lines DL, DL* to a predetermined voltage leveland thereafter activates the word line WL to turn ON the accesstransistor 104. In response to the access transistor 104 turning ON,charge is transferred between the storage capacitor 106 and the digitline DL, causing the voltage on the digit line DL to be slightly higheror lower than the voltage on the digit line DL*. The sense amplifier 108senses the difference between the voltages on the digit lines DL and DL*and drives the voltages on the digit lines to complementary levels inresponse to the sensed difference. For example, assume a voltage VCC/2corresponding to a binary 1 is stored across the capacitor 106. In thissituation, when the access transistor 104 is activated the equilibratedvoltage on the digit line DL will increase slightly relative to theequilibrated voltage on the digit line DL*. As a result, the senseamplifier 108 will drive the voltage on the digit line DL to a supplyvoltage VCC and will drive the complementary digit line DL* to areference voltage. The complementary voltages on the digit lines DL, DL*thus correspond to the data stored in the memory cell 102, and the senseamplifier 108 thereafter applies these signals to other circuitry (notshown) to thereby provide the circuitry with the data stored in thememory cell.

[0006] As previously mentioned, the data stored in the memory cell 102in the form of the voltage across the capacitor 106 must be periodicallyrefreshed. This is true because once the data is stored in the form of avoltage across the capacitor 106 and the access transistor 104 isdeactivated, leakage currents ILK result in this stored voltage changingover time and, if not refreshed, may result in a different binary stateof data being stored in the memory cell. These leakage currents ILKarise, for example, from the flow of charge stored on the conductiveplate 107 of the capacitor 106 through the access transistor 104 evenwhen the access transistor is turned OFF, and may also arise from theflow of charge from the conductive plates 107, 109 to ground, as well asthe flow of charge from the plate 107 through a dielectric (not shown)to the plate 109, as will be appreciated by those skilled in the art.From the above description of the conventional DRAM memory cell 102, itis seen that each time data is read from the memory cell the storagecapacitor 106 is again charged to the proper voltage corresponding tothe data stored in the cell. Thus, to refresh memory cells 102, thememory cells are merely accessed as in a read operation with the senseamplifier 108 driving digit lines DL, DL* to complementary voltagescorresponding to the data stored in the memory cell and thereby chargingthe storage capacitors 106 to the proper voltage.

[0007] The rate at which the data restored in the memory cells 102 mustbe periodically refreshed is known as the refresh rate of the cells, andis a function of a number of different parameters, including theoperating temperature of the DRAM containing the array 100, the numberof rows of memory cells in the array, and the value of the supplyvoltage VCC applied to the DRAM, as will be appreciated by those skilledin the art. For example, if the array 100 includes N rows of memorycells 102 and each memory cell must be refreshed every M milliseconds,the refresh rate is M/N milliseconds/row, meaning that one row must beaccessed every M/N milliseconds in order to properly refresh the memorycells, with every row being accessed at least once every M milliseconds.As the supply voltage VCC decreases, the refresh rate increases due, forexample, to a reduced voltage being stored across the storage capacitors106 and the need to refresh this voltage more frequently to ensure thestored voltage does not decay to an insufficient level due to theleakage currents ILK. The refresh rate also must increase as the supplyvoltage VCC decreases due to the possibility of restoring incorrect datainto the memory cell 102, as will be appreciated by those skilled in theart.

[0008] When the memory-cell array 100 is contained in a DRAM, a memorycontroller typically reads data from desired memory cells 102 inresponse to requests from a microprocessor or other control circuit,each accessed memory cell being automatically refreshed as previouslydescribed. The data stored in all the memory cells 102 and not justthose accessed by the memory controller, however, must be periodicallyrefreshed. As a result, during normal operation the memory controllerwill periodically apply a refresh command to the DRAM containing thearray 100, causing control circuitry (not shown) to access each memorycell 102 as previously described and thereby refreshing the memorycells. Even when the memory controller is not accessing the DRAM, thememory cells 102 must still be periodically refreshed. To refresh thememory cells 102 in this situation, the memory controller applies aself-refresh command to the DRAM, placing the DRAM in a self-refreshmode of operation during which circuitry internal to the DRAM (not shownin FIG. 1) refreshes the memory cells 102 periodically, as will beappreciated by those skilled in the art.

[0009] As previously described, in portable and other electronic devicescontaining DRAM, the supply voltage VCC applied to the DRAM is typicallyreduced during a low-power mode of operation to reduce power consumptionand extend battery life of the device. Notwithstanding the reducedsupply voltage VCC, the memory cells in the DRAM must be adequatelyrefreshed to ensure the integrity of the stored data. There is a needfor an improved circuit and method for controlling the refresh rate ofdynamic data stored in a DRAM or other integrated circuit when thesupply voltage is reduced to a very low level during a low-power mode ofoperation.

SUMMARY OF THE INVENTION

[0010] According to one aspect of the present invention, a method andcircuit for refreshing dynamic data stored in an integrated circuit aredisclosed. The integrated circuit receives a supply voltage and operatesin a self-refresh mode of operation to refresh the dynamic data at arefresh time that defines how often the dynamic data is refreshed duringthe self-refresh mode. The method includes monitoring a magnitude of thesupply voltage and adjusting the refresh time as a function of themonitored magnitude of the supply voltage. The integrated circuit may beany type of integrated circuit that stores dynamic data, such as amemory device like a DRAM, double-data rate (DDR) DRAM, SLDRAM, RDRAM,or other type of integrated circuit such as a microprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a diagram illustrating a portion of a memory-cell arrayin a conventional DRAM.

[0012]FIG. 2 is a functional block diagram of a memory system includinga memory controller and a memory device including a self-refreshcontroller according to one embodiment of the present invention.

[0013]FIGS. 3A and 3B are diagrams illustrating signals generated by theself-refresh controller of FIG. 1 in controlling the frequency of aclock signal and thereby controlling a refresh rate of memory cells as afunction of a supply voltage according to a first embodiment of thepresent invention.

[0014]FIG. 4 is a signal diagram illustrating the operation of theself-refresh controller of FIG. 1 in controlling the frequency of aclock signal and thereby controlling a refresh rate of memory cells as afunction of the supply voltage according to a second embodiment of thepresent invention.

[0015]FIG. 5 is a signal diagram illustrating the operation of theself-refresh controller of FIG. 1 in controlling the frequency of aclock signal and thereby controlling a refresh rate of memory cells as afunction of the supply voltage according to a third embodiment of thepresent invention.

[0016]FIG. 6 is a functional block diagram illustrating a computersystem including the memory device of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

[0017]FIG. 2 is a functional block diagram of a memory system 200including a memory controller 202 coupled to a memory device 204 thatincludes a self-refresh controller 206 for adjusting the refresh rate ofdynamic data as a function of an applied supply voltage VCC according toone embodiment of the present invention. In operation, the self-refreshcontroller 206 adjusts a refresh frequency RF of a refresh clock signalRFCLK, which defines a refresh rate of the dynamic data as a function ofthe supply voltage VCC, to ensure the integrity of data as the supplyvoltage decreases, as will be explained in more detail below. The memorydevice 204 in FIG. 2 is a double-data rate (DDR) synchronous dynamicrandom access memory (“SDRAM”), although the principles described hereinare applicable to any memory device containing memory cells that must berefreshed (i.e., that store dynamic data), such as conventional DRAMsand SDRAMs, as well as packetized memory device like SLDRAMs and RDRAMs,and are equally applicable to any integrated circuit that stores dynamicdata. In the following description, certain details are set forth toprovide a sufficient understanding of the invention. It will be clear toone skilled in the art, however, that the invention may be practicedwithout these particular details. In other instances, well-knowncircuits, control signals, timing protocols, and software operationshave not been shown in detail or omitted entirely in order to avoidunnecessarily obscuring the invention.

[0018] Before describing the self-refresh controller 206 in more detail,the various components of the memory device 204 will first be described.The memory controller 202 applies row, column, and bank addresses to anaddress register 208 over an address bus ADDR. Typically, a row addressRA and a bank address BA are initially received by the address register208 and applied to a row address multiplexer 208 and bank control logiccircuit 210, respectively. The row address multiplexer 208 applieseither the row address RA received from the address register 208 or arefresh row address RFRA received from the self-refresh controller 206to a plurality of row address latch and decoder circuits 214A-D. Thebank control logic 212 activates the row address latch and decodercircuit 214A-D corresponding to either the received bank address BA or arefresh bank address RFBA from the self-refresh controller 206, and theactivated row address latch and decoder circuit latches and decodes thereceived row address. In response to the decoded row address, theactivated row address latch and decoder 214A-D applies various signalsto a corresponding memory bank or array 216A-D to thereby activate a rowof memory cells corresponding to the decoded row address. The data inthe memory cells in the accessed row is stored in sense amplifierscoupled to the array 216A-D, which also refreshes the accessed memorycells as previously described. The row address multiplexer 210 appliesthe refresh row address RFRA to the row address latch and decoders214A-D and the bank control logic circuit 212 uses the refresh bankaddress RFBA when the memory device 204 operates in an auto-refresh orself-refresh mode of operation in response to the controller 202applying an auto- or self-refresh command to the memory device 204, aswill be described in more detail below.

[0019] After the address register 208 memory controller 202 has appliedthe row and bank addresses RA, BA, the memory controller applies acolumn address CA on the address bus ADDR. The address register 208provides the column address CA to a column address counter and latchcircuit 218 which, in turn, latches the column address and applies thelatched column address to a plurality of column decoders 220A-D. Thebank control logic 212 activates the column decoder 220A-D correspondingto the received bank address BA, and the activated column decoderdecodes the column address CA from the counter and, latch circuit 218.Depending on the operating mode of the memory device 204, the counterand latch circuit 218 either directly applies the latched column addressto the decoders 220A-D, or applies a sequence of column addresses to thedecoders starting at the column address CA provided by the addressregister 208. In response to the column address from the counter andlatch circuit 218, the activated column decoder 222A-D applies decodeand control signals to an I/O gating and data masking circuit 222 which,in turn, accesses memory cells corresponding to the decoded columnaddress in the activated row of memory cells in the array 216A-D beingaccessed.

[0020] During data read operations, data being read from the activatedarray 216A-D is coupled through the I/O gating and data masking circuit222 to a read latch 224. The circuit 222 supplies N bits of data to theread latch 224, which then applies two N/2 bit words to a multiplexer226. In the embodiment of FIG. 3, the circuit 222 provides 64 bits tothe read latch 224 which, in turn, provides two 32 bits words to themultiplexer 226. A data driver circuit 228 sequentially receives the N/2bit words from the multiplexer 226 and also receives a data strobesignal DQS from a strobe signal generator 230 and a delayed clock signalCLKDEL from a delay-locked loop (DLL) circuit 232. The DQS signal hasthe same frequency as the CLK, CLK* signals, and is used by thecontroller 202 in latching data from the memory device 204 during readoperations, as will be described in more detail below. In response tothe delayed clock signal CLKDEL, the data driver circuit 228sequentially outputs the received N/2 bits words as corresponding datawords DQ that are in synchronism with rising and falling edges of theCLK signal, respectively, and also outputs the data strobe signal DQShaving rising and falling edges in synchronism with rising and fallingedges of the CLK signal, respectively. Each data word DQ and the datastrobe signal DQS collectively define a data bus DATA coupled to thecontroller 202 which, during read operations, latches the each N/2 bitDQ word on the DATA bus responsive to the data strobe signal DQS. Aswill be appreciated by those skilled in the art, the CLKDEL signal is adelayed version of the CLK signal, and the DLL circuit 232 adjusts thedelay of the CLKDEL signal relative to the CLK signal to ensure that theDQS signal and the DQ words are placed on the DATA bus in synchronismwith the CLK signal. The DATA bus also includes masking signals DQM0-X,which will be described in more detail below with reference to datawrite operations.

[0021] During data write operations, the memory controller 202 appliesN/2 bit data words DQ, the strobe signal DQS, and corresponding datamasking signals DM0-X on the data bus DATA. A data receiver circuit 234receives each DQ word and the associated DM0-X signals, and appliesthese to an input register 236 that is clocked by the DQS signal. Inresponse to a rising edge of the DQS signal, the input register 236latches a first N/2 bit DQ word and the associated DM0-X signals, and inresponse to a falling edge of the DQS signal the input register latchesthe corresponding N/2 bit DQ word and associated DM0-X signals. Theinput register 236 provides the two latched N/2 bit DQ words as an N-bitword to a write FIFO and driver circuit 238, which clocks the theapplied DQ word and DM0-X signals into the write FIFO and driver circuitin response to the DQS signal. The DQ word is clocked out of the writeFIFO and driver circuit 238 in response to the CLK signal, and isapplied to the I/O gating and masking circuit 222. The I/O gating andmasking circuit 222 transfers the DQ word to the accessed memory cellsin the activated array 216A-D subject to the DM0-X signals, which may beused to selectively mask bits or groups of bits in the DQ words (i.e.,in the write data) being written to the accessed memory cells.

[0022] A control logic and command decoder circuit 240 receives aplurality of command and clocking signals from the memory controller 202over a control bus CONT, and generates a plurality of control and timingsignals to control the components 206-238 during operation of the memorydevice 204. The command signals include a chip select signal CS*, awrite enable signal WE*, a column address strobe signal CAS*, and a rowaddress strobe signal RAS*, while the clocking signals include a clockenable signal CKE* and complementary clock signals CLK, CLK*, with the“*” designating a signal as being active low. The memory controller 202drives the command signals CS*, WE*, CAS*, and RAS* to valuescorresponding to a particular command, such as a read, write, orauto-refresh command. In response to the clock signals CLK, CLK*, thecommand decoder circuit 240 latches and decodes an applied command, andgenerates a sequence of control signals that control various componentsin the memory device to execute the function of the applied command. Theclock enable signal CKE enables clocking of the command decoder circuit240 by the clock signals CLK, CLK*. The command decoder circuit 240latches command and address signals at positive edges of the CLK, CLK*signals (i.e., the crossing point of CLK going high and CLK* going low),while the input registers 236 and data drivers 228 transfer data intoand from, respectively, the memory device 204 in response to both edgesof the data strobe signal DQS and thus at double the frequency of thestrobe signal and clock signals CLK, CLK*. For this reason the memorydevice 204 is referred to as a double-data-rate device, with data beingtransferred to and from the device at double the rate of a conventionalSDRAM, which transfers data at a rate corresponding to the frequency ofthe applied clock signal. The detailed operation of the control logicand command generator circuit 240 in generating the control and timingsignals is conventional, and thus, for the sake of brevity, will not bedescribed in more detail.

[0023] As previously mentioned, in battery-powered electronic devices itis desirable to place the memory device 204 in a low-power mode ofoperation when the memory controller 202 is not accessing data stored inthe memory device. In the memory device 204, this low-power mode ofoperation is known as a self-refresh mode. To place the memory device204 in the self-refresh mode of operation, the memory controller 202applies a self-refresh command to the memory device. In response to theself-refresh command, the command decoder circuit 240 applies controlsignals to the row address multiplexer 210 and the bank control logiccircuit 212 that cause the circuits to utilize the refresh row addressRFRA and refresh bank address RFBA from the self-refresh controller 206to sequentially access each row of memory cells in the memory array216A-D to thereby refresh the memory cells. The self-refresh controller206 controls the refresh rate at which the memory cells in the arrays216A-D0 are refreshed as a function of a supply voltage VCC applied tothe memory device 204. The operation of the self-refresh controller 206during the self-refresh mode along with the structure of theself-refresh controller will now be described in more detail.

[0024] The self-refresh controller 206 includes a bias voltage generator242 that receives the supply voltage VCC and generates a bias voltageVBIAS having a value that is a function of the magnitude of the supplyvoltage. A self-refresh oscillator 244 receives the bias voltage VBIASand generates a refresh clock signal RFCLK having a refresh frequency RFthat is a function of the bias voltage VBIAS. The self-refreshoscillator 244 applies the refresh clock signal RFCLK to clock aself-refresh row-bank address counter 246 which sequentially generatesthe refresh row addresses RFRA and bank addresses RFBA in response tothe RFCLK signal, and applies the refresh row address to the row addressmultiplexer 210 and refresh bank address to the bank control logiccircuit 212 as previously described.

[0025] In operation, upon receiving a self-refresh command, the controllogic and command decoder circuit 240 resets the counter 246 and appliescontrol signals causing the row address multiplexer 210 and bank controllogic circuit 212 to utilize the refreshed row address RFRA and refreshbank address RFBA, respectively. The self-refresh oscillator 244 appliesthe refresh clock signal RFCLK to clock the counter 246 which, in turn,sequentially generates the refresh row addresses RFRA and refresh bankaddresses RFBA. The sequentially generated refresh row addresses RFRAare applied through the multiplexer 210 and latched and decoded by theactivated row address latch and decoder circuit 214A-D, with the bankcontrol logic circuit 212 activating the circuit 214A-D corresponding tothe refresh bank address RFBA. The refresh controller 206 generates agiven refresh bank address RFBA and then generates refresh row addressesRFRA to sequentially activate all rows in the memory array 216A-Dcorresponding to the bank address, and thereafter generates a new bankaddress and activates each row in the new memory array, and so on foreach memory array. In this way, the refresh controller 206 sequentiallyactivates rows of memory cells in the arrays 216A-D to thereby refreshthe memory cells. Although the refresh controller 206 is discussed asgenerating addresses that refresh the memory cells during theself-refresh mode, one skilled in the art will appreciate that thecontrol logic and command decoder circuit 240 also generates signals tocontrol various components in the memory device 204 during this mode ofoperation.

[0026] The refresh rate of the memory cells in the arrays 216A-D isdetermined by the rate at which the counter 246 sequentially generatesthe refresh row and bank addresses RFRA, RFBA, which is determined bythe frequency RF of the applied refresh clock signal RFCLK, as will beappreciated by those skilled in the art. Thus, the frequency RF of theRFCLK clock signal determines the refresh rate of the memory cells inthe arrays 216A-D. As previously mentioned, the frequency RF of theRFCLK signal is a function of the bias voltage VBIAS from the variablebias voltage generator 242, and the bias voltage is a function of themagnitude of the supply voltage VCC. The refresh rate of the memorycells in the arrays 216A-D is therefore a function the magnitude of thesupply voltage VCC. In this way, the self refresh controller 206controls the refresh rate as a function of the supply voltage VCC toensure the refresh rate is sufficient to reliably maintain the datastored in the arrays 216A′-D. For example, as the supply voltage VCCdecreases during a low-power mode of operation, the self-refreshcontroller 206 increases the refresh rate of the memory cells in thearrays 216A-D to ensure data integrity.

[0027] In the self-refresh controller 206, the variable bias voltagegenerator 242 controls the bias voltage VBIAS as a function of themagnitude of the supply voltage VCC, and the bias voltage is applied tothe self-refresh oscillator 244 to control the frequency RF of the RFCLKsignal and thereby control the refresh rate of the memory cells in thearrays 216A-D as a function of the supply voltage VCC. Accordingly, theprecise manner in which the variable bias voltage generator 242 controlsthe bias voltage VBIAS as a function of the supply voltage VCC and theprecise manner in which the self-refresh oscillator 244 controls thefrequency RF of the RFCLK signal in response to the bias voltagedetermine how the self refresh controller 206 controls the refresh rateas the supply voltage varies.

[0028]FIGS. 3A and 3B are signal diagrams illustrating the operation ofthe variable bias voltage generator 242 and self-refresh oscillator 244in combination to control the frequency RF of the RFCLK signal as afunction of the supply voltage VCC according to one embodiment of thepresent invention. In the embodiment of FIG. 3A, the variable biasvoltage generator 242 maintains the bias voltage VBIAS at a relativelyconstant value VBC when the supply voltage VCC is greater than a minimumvalue VMIN. As seen in FIG. 3B, the relatively constant bias voltage VBCwhen the supply voltage VCC is greater than the voltage VMIN results inthe oscillator 244 developing the RFCLK signal have a relativelyconstant frequency RFN. The supply voltage VCC being greater than theminimum value VMIN corresponds to a normal operating mode of the memorydevice 204. When the supply voltage VCC is less than or equal to theminimum value VMIN, the variable bias voltage generator 242 beginsincreasing the bias voltage VBIAS as the supply voltage decreases, whichincreases the frequency RF of the RFCLK signal and thereby increases therefresh rate of the memory cells in the arrays 216A-D. The supplyvoltage VCC being less than or equal to the minimum value VMIN andgreater than a lower limit VL corresponds to a low-power operating modeof the memory device 204. Thus, in the embodiment of FIGS. 3A and 3B,the refresh rate is increased as the supply voltage VCC decreases belowa minimum value VMIN to ensure data is adequately refresh during alow-power mode of operation. The lower limit VL corresponds to a supplyvoltage VCC having such a small magnitude that the refresh controller206 can no longer reliably refresh data stored in the memory arrays216A-D.

[0029]FIG. 4 is a signal diagram illustrating the operation of thevariable bias voltage generator 242 and the self-refresh oscillator 244in combination to control the frequency RF of the RFCLK signal as afunction of the supply voltage VCC according to a second embodiment ofthe present invention. In the embodiment of FIG. 4, the variable biasvoltage generator 242 maintains the bias voltage VBIAS relativelyconstant when the supply voltage VCC is greater than a minimum valueVMIN, resulting in the oscillator 244 maintaining the frequency RF ofthe RFCLK at a relatively constant value RFN. Once again, when thesupply voltage VCC is greater than the minimum value VMIN the memorydevice 204 operates in a normal operating mode. In this embodiment, whenthe variable bias voltage generator 242 detects the supply voltage VCCis less than or equal to the minimum value VMIN, the voltage generatoroutputs the supply voltage as the bias voltage VBIAS. As seen in FIG. 4,when the supply voltage VCC is output as the bias voltage VBIAS, thefrequency RF of the RFCLK signal increases to a maximum value RFM due tothe increased magnitude of the bias voltage, and the refresh rateincreases accordingly. The frequency RF and, accordingly, the refreshrate thereafter decrease as the supply voltage VCC and thus the biasvoltage VBIAS decrease. The supply voltage VCC being less than or equalto the minimum value VMIN and greater than a lower limit VL once againcorresponds to a low-power operating mode of the memory device 204. Inthe embodiment of FIG. 4, the magnitude of the bias voltage VBIAS isincreased due to the greater magnitude of the supply voltage VCC, whichis applied as the bias voltage. This increased bias voltage VBIASincreases the frequency RF of the RFCLK signal which, in turn, increasesthe refresh rate of the memory cells in the arrays 216A-D.

[0030]FIG. 5 is a signal diagram illustrating the operation of thevariable bias voltage generator 242 and the self-refresh oscillator 244in combination to control the frequency RF of the RFCLK signal as afunction of the supply voltage VCC according to a third embodiment ofthe present invention. In this embodiment, the variable bias voltagegenerator 242 maintains the bias voltage VBIAS at a relatively constantvalue VBC when the supply voltage VCC is greater than a minimum valueVMIN to thereby cause the oscillator 244 to develop the RFCLK signalhave a relatively constant frequency RFN. The supply voltage VCC beinggreater than the minimum value VMIN once again corresponds to a normaloperating mode of the memory device 204. When the supply voltage VCC isless than or equal to the minimum value VMIN, the variable bias voltagegenerator 242 begins increasing the bias voltage VBIAS as the supplyvoltage decreases, which increases the frequency RF of the RFCLK signaland thereby increases the refresh rate of the memory cells in the arrays216A-D. The variable bias voltage generator 242 and oscillator 244operate in this way, which corresponds to the operation previouslydescribed with reference to FIGS. 3A and 3B, to increase the refreshrate as the supply voltage VCC decreases.

[0031] The generator 242 and oscillator 244 operate in this manner untilthe variable bias voltage generator 242 detects the supply voltage VCCis less than a first lower limit VF. When the variable bias voltagegenerator 242 determines the supply voltage VCC is less than or equal tothe first lower limit VF, the bias voltage generator operates aspreviously described with reference to FIG. 4, outputting the supplyvoltage as the bias voltage VBIAS to the oscillator 244. The increasedmagnitude of the supply voltage VCC being output as the bias voltageVBIAS causes the frequency RF of the RFCLK signal to increase to amaximum value RFM, and the refresh rate increases accordingly. Thefrequency RF and, accordingly, the refresh rate thereafter decrease asthe supply voltage VCC and thus the bias voltage VBIAS decrease. Thesupply voltage VCC being less than or equal to the minimum value VMINand greater than a second lower limit VL once again corresponds to alow-power operating mode of the memory device 204. It should be notedthat in this embodiment, the low-power operating mode includes two submodes, a first submode corresponding to the operation of the generator242 and oscillator 244 when the supply voltage VCC is between theminimum value VMIN and the first lower limit VF, and a second sub modewhen the supply voltage is between the first lower limit VF and thesecond lower limit VL.

[0032] Referring back to FIG. 2, in another embodiment of the refreshcontroller 206, the memory controller 202 monitors the supply voltageVCC. When the memory controller 202 determines the supply voltage VCC isless than a minimum value VMIN, the memory controller applies a refreshrate adjustment command to the memory device 204. This refresh rateadjustment command may, for example, correspond to a load mode commandthat loads appropriate information into mode registers contained in thecontrol logic and command decode circuit 240. In response to receivingthe refresh rate adjustment command, the command decode circuit 240applies control signals to the variable bias voltage generator 242,causing the voltage generator to operate as previously described for theembodiment of FIGS. 3A-3B. In this embodiment, the memory controller 202could also further monitor the supply voltage VCC and send anotherrefresh rate adjustment command to the memory device 204 when the supplyvoltage becomes less than a first lower limit VF, with the commanddecode circuit 240 thereafter causing the variable bias voltagegenerator 242 and oscillator 244 to operate as previously described forthe embodiment of FIG. 5 when the supply voltage is between the firstlower limit VF and the second lower limit VL. When the memory controller202 applies this second refresh rate adjustment command, the refreshcontroller 202 operates the same as in the embodiment of FIG. 4.

[0033] As will be appreciated by those skilled in the art, otherembodiments of the refresh controller 206 in which the refreshcontroller controls the refresh rate in different ways as a function ofthe magnitude of the supply voltage VCC are well within the scope of thepresent invention.

[0034]FIG. 6 is a block diagram of a computer system 700 includingcomputer circuitry 702 which includes the memory device 204 of FIG. 2,and which may also include of the memory controller 202 of FIG. 2 aswell. Typically, the computer circuitry 702 is coupled to the memorycontroller 202 through address, data, and control buses to provide forwriting data to and reading data from the memory device 204. Thecomputer circuitry 702 includes circuitry for performing variouscomputing functions, such as executing specific software to performspecific calculations or tasks. In addition, the computer system 700includes one or more input devices 704, such as a keyboard or a mouse,coupled to the computer circuitry 702 to allow an operator to interfacewith the computer system. Typically, the computer system 700 alsoincludes one or more output devices 706 coupled to the computercircuitry 702, such as output devices typically including a printer anda video terminal. One or more data storage devices 708 are alsotypically coupled to the computer circuitry 702 to store data orretrieve data from external storage media (not shown). Examples oftypical storage devices 708 include hard and floppy disks, tapecassettes, compact disk read-only (CD-ROMs) and compact disk read-write(CD-RW) memories, and digital video disks (DVDs).

[0035] It is to be understood that even though various embodiments andadvantages of the present invention have been set forth in the foregoingdescription, the above disclosure is illustrative only, and changes maybe made in detail, and yet remain within the broad principles of theinvention. For example, many of the components described above may beimplemented using either digital or analog circuitry, or a combinationof both, and also, where appropriate, may be realized through softwareexecuting on suitable processing circuitry. Therefore, the presentinvention is to be limited only by the appended claims.

1. A method of refreshing dynamic data stored in an integrated circuit,the integrated circuit being adapted to receive a supply voltage andbeing adapted to operate in a self-refresh mode of operation to refreshthe dynamic data at a refresh time that defines how often the dynamicdata is refreshed during the self-refresh mode, the method comprisingmonitoring a magnitude of the supply voltage and adjusting the refreshtime as a function of the monitored magnitude of the supply voltage. 2.The method of claim 1 wherein adjusting the refresh time as a functionof the monitored magnitude of the supply voltage comprises decreasingthe refresh time as the magnitude of the supply voltage decreases torefresh the dynamic data more frequently.
 3. The method of claim 2wherein decreasing the refresh time as the magnitude of the supplyvoltage decreases comprises linearly increasing a frequency of aself-refresh clock signal, the self-refresh clock signal defining therefresh time.
 4. The method of claim 1 wherein monitoring the magnitudeof the supply voltage comprises detecting whether the magnitude is lessthan or equal to a minimum supply voltage.
 5. The method of claim 4wherein adjusting the refresh time as a function of the monitoredmagnitude of the supply voltage comprises reducing the refresh time oncethe supply voltage has been detected as being less than or equal to theminimum supply voltage.
 6. The method of claim 1 wherein monitoring themagnitude of the supply voltage comprises detecting whether the supplyvoltage is less than or equal to a minimum supply voltage, and adjustingthe refresh time as a function of the monitored magnitude of the supplyvoltage comprises maintaining the refresh time substantially constantuntil the supply voltage is less than or equal to the minimum supplyvoltage, and decreasing the refresh time as a function of decreases inthe supply voltage once the detected supply voltage is less than orequal to the minimum supply voltage.
 7. The method of claim 6 whereindecreasing the refresh time as a function decreases in the supplyvoltage once the detected supply voltage is less than or equal to theminimum supply voltage comprises generating a bias voltage from thesupply voltage, the bias voltage having a value that is inverselyrelated to the refresh time, and increasing the bias voltage as afunction of decreases in the supply voltage once the detected supplyvoltage is less than or equal to the minimum supply voltage.
 8. Themethod of claim 1 wherein monitoring the magnitude of the supply voltagecomprises detecting whether the supply voltage is less than or equal toa minimum supply voltage, and adjusting the refresh time as a functionof the monitored magnitude of the supply voltage comprises generating arefresh time adjustment command that determines a value of the refreshtime, the value being a function of the detected supply voltage, andsetting the refresh time to the determined value.
 9. The method ofclaims 8 wherein generating a refresh time adjustment command comprisesgenerating the command external to the integrated circuit and applyingthe command to the integrated circuit, and setting the refresh time tothe determined value comprises decoding the applied command andadjusting the refresh time to the value corresponding to the decodedcommand.
 10. A method of refreshing dynamic data stored in an integratedcircuit, the integrated circuit being adapted to receive a supplyvoltage and including a refresh oscillator that generates a refreshclock signal having a frequency that is a function of a bias voltage,the bias voltage being derived from the supply voltage and theintegrated circuit being adapted to operate in a self-refresh mode ofoperation to refresh the dynamic data at a refresh rate that is afunction of the frequency of the refresh clock signal, the methodcomprising monitoring the supply voltage and controlling the biasvoltage as a function of the monitored supply voltage.
 11. The method ofclaim 10 wherein controlling the bias voltage as a function of themonitored supply voltage comprises maintaining the bias voltagesubstantially constant when the supply voltage is greater than a minimumsupply voltage, and adjusting the bias voltage as the supply voltagedecreases when the supply monitored supply voltage is less than or equalto the minimum supply voltage.
 12. The method of claim 11 whereinmonitoring the supply voltage comprises detecting whether the supplyvoltage is less than or equal to a minimum supply voltage, andcontrolling the bias voltage as a function of the monitored supplyvoltage comprises maintaining the bias voltage substantially constantwhen the supply voltage is greater than the minimum supply voltage andadjusting the bias voltage to increase the frequency of the refreshclock signal as a linear function of decreases in the supply voltagewhen the detected supply voltage is less than or equal to the minimumsupply voltage.
 13. The method of claim 10 wherein controlling the biasvoltage as a function of the monitored supply voltage comprisesmaintaining the bias voltage substantially constant when the supplyvoltage is greater than a minimum supply voltage and applying the supplyvoltage as the bias voltage when the supply voltage is less than orequal to the minimum supply voltage.
 14. The method of claim 10 whereincontrolling the bias voltage as a function of the monitored supplyvoltage comprises maintaining the bias voltage substantially constantwhen the supply voltage is greater than a first minimum supply voltage,and adjusting the bias voltage to increase the frequency of the refreshclock signal as a function of decreases in the supply voltage when thesupply voltage is less than or equal to the first minimum supply voltageand greater than a second minimum supply voltage, and applying thesupply voltage as the bias voltage when the supply voltage is less thanor equal to the second minimum supply voltage.
 15. The method of claim10 wherein monitoring the supply voltage comprises detecting whether thesupply voltage is less than or equal to a minimum supply voltage, andcontrolling the bias voltage as a function of the monitored supplyvoltage comprises generating a bias voltage adjustment command, thecommand having a first value that maintains the bias voltagesubstantially constant when the supply voltage is greater than theminimum supply voltage, and generating the bias voltage adjustmentcommand having a second value when the supply voltage is less than orequal to the minimum supply voltage, and adjusting the bias voltage as afunction of the supply voltage when the command has the second value.16. The method of claims 15 wherein generating a bias voltage adjustmentcommand comprises generating the bias voltage adjustment commandexternal to the integrated circuit and applying the command to theintegrated circuit, and adjusting the bias voltage as a function of thesupply voltage when the bias voltage adjustment command has the secondvalue comprises decoding the applied bias voltage adjustment command andthereafter adjusting the bias voltage as the supply voltage varies. 17.A refresh controller for controlling a refresh rate of dynamic data, therefresh controller being adapted to receive a supply voltage andincluding a refresh oscillator that generates a refresh clock signalhaving a frequency that is a function of a bias voltage, the refreshcircuit operable in a normal mode when the supply voltage is greaterthan a minimum value to apply a substantially constant bias voltage tothe refresh oscillator and to generate refresh signals in response tothe corresponding refresh clock signal, the refresh signals defining afirst refresh rate having a value determined by the frequency of therefresh clock signal, and the refresh controller operable in a low-powermode when the supply voltage is less than or equal to the minimum valueto vary the bias voltage as a function of the supply voltage and togenerate refresh signals in response to the corresponding refresh clocksignal, the refresh signals defining a second refresh rate having avariable value that is a function of the frequency of the refresh clocksignal.
 18. The refresh controller of claim 17 wherein the refreshcontroller operates in the low-power mode to vary the bias voltage as afunction of the supply voltage to linearly increase the frequency of therefresh clock signal as the supply voltage decreases below the minimumvalue.
 19. The refresh controller of claim 17 wherein the refreshcontroller generates the bias voltage to develop the refresh clocksignal having a substantially constant frequency in the normal mode andgenerates the bias voltage to increase the frequency of the refreshclock signal as the supply voltage decreases in the low-power mode. 20.The refresh controller of claim 17 wherein the refresh controllergenerates the bias voltage to develop the refresh clock signal having asubstantially constant frequency in the normal mode and outputs thesupply voltage as the bias voltage to directly control the frequency ofthe refresh clock signal as a function of the value of the supplyvoltage in the low-power mode.
 21. The refresh controller of claim 17wherein the refresh controller generates the bias voltage to develop therefresh clock signal having a substantially constant frequency in thenormal mode, and operates in the low-power mode to generate the biasvoltage to increase the frequency of the refresh clock signal as thesupply voltage decreases when the supply voltage is less than or equalto the minimum value and greater than a second minimum value, andfurther operates in the low-power mode to output the supply voltage asthe bias voltage to directly control the frequency of the refresh clocksignal as a function of the value of the supply voltage when the supplyvoltage is less than or equal to the second minimum supply value. 22.The refresh controller of claim 17 wherein the integrated circuit isadapted to receive a bias voltage adjustment command that is generatedwhen the refresh controller is to operate in the low-power mode, thecommand being decoded and applied to the refresh controller which, inresponse to the decoded command, generates the bias voltage to increasethe frequency of the refresh clock signal as the supply voltagedecreases.
 23. The refresh controller of claim 17 wherein the integratedcircuit comprises a dynamic random access memory.
 24. A refreshcontroller for controlling a refresh rate of dynamic data, the refreshcontroller comprising: a variable bias voltage generator adapted toreceive a supply voltage and operable in a normal mode to generate abias voltage having a substantially constant value, and operable in alow-power mode to adjust the value of the bias voltage as a function ofa magnitude of the supply voltage; a self-refresh oscillator coupled tothe variable bias voltage generator to receive the bias voltage signal,the self-refresh oscillator developing a refresh clock signal having afrequency that is a function of the bias voltage; and a refresh controlcircuit coupled to the self-refresh oscillator to receive the refreshclock signal, the refresh control circuit operable in response to therefresh clock signal to develop refresh signals that control the refreshrate of the dynamic data as a function of the frequency of the refreshclock signal.
 25. The refresh controller of claim 24 wherein thevariable bias voltage generator is operable in the normal moderesponsive to the supply voltage being greater than a minimum value, andis operable in the low-power mode responsive to the supply voltage beingless than or equal to the minimum value.
 26. The refresh controller ofclaim 24 wherein the bias voltage generator operates in the low-powermode to adjust the bias voltage as a linear function of the supplyvoltage.
 27. The refresh controller of claim 24 wherein the self-refreshoscillator increases the frequency of the refresh clock signal as thebias voltage decreases and decreases the frequency of the refresh clocksignal as the bias voltage increases.
 28. The refresh controller ofclaim 24 wherein the refresh control circuit comprises a self-refreshrow and bank address counter operable to generate row and bank addressesresponsive to the refresh clock signal, the rate at which the row andbank addresses are generated being a function of the frequency of therefresh clock signal and the refresh rate being a function of the rateat which the row and bank addresses are generated.
 29. A refreshcontroller for controlling a refresh rate of dynamic data, the refreshcontroller comprising: a variable bias voltage generator adapted toreceive a supply voltage and operable in a normal mode to generate abias voltage having a substantially constant value, and operable in alow-power mode to output the supply voltage as the bias voltage; aself-refresh oscillator coupled to the variable bias voltage generatorto receive the bias voltage signal, the self-refresh oscillatordeveloping a refresh clock signal having a frequency that is a functionof the bias voltage; and a refresh control circuit coupled to theself-refresh oscillator to receive the refresh clock signal, the refreshcontrol circuit operable in response to the refresh clock signal todevelop refresh signals that control the refresh rate of the dynamicdata as a function of the frequency of the refresh clock signal.
 30. Therefresh controller of claim 29 wherein the voltage bias generatoroperates in the normal mode responsive to the supply voltage beinggreater than a minimum value, and operates in the low-power moderesponsive to the supply voltage being less than or equal to the minimumvalue.
 31. The refresh controller of claim 29 wherein the self-refreshoscillator increases the frequency of the refresh clock signal as thebias voltage increases and decreases the frequency of the refresh clocksignal as the bias voltage decreases.
 32. The refresh controller ofclaim 29 wherein the refresh control circuit comprises a self-refreshrow and bank address counter operable to generate row and bank addressesresponsive to the refresh clock signal, the rate at which the row andbank addresses are generated being a function of the frequency of therefresh clock signal and the refresh rate being a function of the rateat which the row and bank addresses are generated.
 33. A refreshcontroller for controlling a refresh rate of dynamic data in anintegrated circuit, the refresh controller comprising: a variable biasvoltage generator adapted to receive a supply voltage and operable in anormal mode to generate a bias voltage having a substantially constantvalue, and operable in a low-power mode responsive to a bias voltageadjustment command that is generated external to the integrated circuit,the variable bias voltage generator operable in response to the commandto adjust the value of the bias voltage as a function of a magnitude ofthe supply voltage; a self-refresh oscillator coupled to the variablebias voltage generator to receive the bias voltage signal, theself-refresh oscillator developing a refresh clock signal having afrequency that is a function of the bias voltage; and a refresh controlcircuit coupled to the self-refresh oscillator to receive the refreshclock signal, the refresh control circuit operable in response to therefresh clock signal to develop refresh signals that control the refreshrate of the dynamic data as a function of the frequency of the refreshclock signal.
 34. The refresh controller of claim 33 wherein biasvoltage adjustment command comprises a load mode register command. 35.The refresh controller of claim 33 wherein the variable bias voltagegenerator is operable in the normal mode responsive to the supplyvoltage being greater than a minimum value, and is operable in thelow-power mode responsive to the supply voltage being less than or equalto the minimum value.
 36. The refresh controller of claim 33 wherein thebias voltage generator operates in the low-power mode to adjust the biasvoltage as a linear function of the supply voltage.
 37. The refreshcontroller of claim 33 wherein the self-refresh oscillator increases thefrequency of the refresh clock signal as the bias voltage decreases anddecreases the frequency of the refresh clock signal as the bias voltageincreases.
 38. The refresh controller of claim 33 wherein the refreshcontrol circuit comprises a self-refresh row and bank address counteroperable to generate row and bank addresses responsive to the refreshclock signal, the rate at which the row and bank addresses are generatedbeing a function of the frequency of the refresh clock signal and therefresh rate being a function of the rate at which the row and bankaddresses are generated.
 39. A refresh controller for controlling arefresh rate of dynamic data, the refresh controller comprising: avariable bias voltage generator adapted to receive a supply voltage, thevariable bias voltage generator operable in a normal mode to generate abias voltage having a substantially constant value, and operable in afirst low-power mode to adjust the value of the bias voltage as afunction of a magnitude of the supply voltage and operable in a secondlow-power mode to output the supply voltage as the bias voltage; aself-refresh oscillator coupled to the variable bias voltage generatorto receive the bias voltage signal, the self-refresh oscillatordeveloping a refresh clock signal having a frequency that is a functionof the bias voltage; and a refresh control circuit coupled to theself-refresh oscillator to receive the refresh clock signal, the refreshcontrol circuit operable in response to the refresh clock signal todevelop refresh signals that control the refresh rate of the dynamicdata as a function of the frequency of the refresh clock signal.
 40. Therefresh controller of claim 39 wherein the variable bias voltagegenerator is operable in the normal mode responsive to the supplyvoltage being greater than a first minimum value, and is operable in thefirst low-power mode responsive to the supply voltage being less than orequal to the first minimum value and greater than a second minimumvalue, and is operable in the second low-power mode responsive to thesupply voltage being less than or equal to the second minimum value. 41.The refresh controller of claim 40 wherein the bias voltage generatoroperates in the first low-power mode to adjust the bias voltage as alinear function of the supply voltage.
 42. The refresh controller ofclaim 40 wherein the self-refresh oscillator increases the frequency ofthe refresh clock signal as the bias voltage increases and decreases thefrequency of the refresh clock signal as the bias voltage decreases. 43.The refresh controller of claim 40 wherein the refresh control circuitcomprises a self-refresh row and bank address counter operable togenerate row and bank addresses responsive to the refresh clock signal,the rate at which the row and bank addresses are generated being afunction of the frequency of the refresh clock signal and the refreshrate being a function of the rate at which the row and bank addressesare generated.
 44. A memory device, comprising: an address decodercoupled to an address bus; a read/write circuit coupled to a data bus; acontrol circuit coupled to a control bus; a memory-cell array coupled tothe address decoder, control circuit, and read/write circuit, thememory-cell array storing dynamic data; and a refresh controller coupledto the memory-cell array, control circuit and address decoder forcontrolling a refresh rate of the dynamic data, the refresh controllerbeing adapted to receive a supply voltage and including a refreshoscillator that generates a refresh clock signal having a frequency thatis a function of a bias voltage, the refresh circuit operable in anormal mode when the supply voltage is greater than a minimum value toapply a substantially constant bias voltage to the refresh oscillatorand to generate refresh signals in response to the corresponding refreshclock signal, the refresh signals defining a first refresh rate having avalue determined by the frequency of the refresh clock signal, and therefresh controller operable in a low-power mode when the supply voltageis less than or equal to the minimum value to vary the bias voltage as afunction of the supply voltage and to generate refresh signals inresponse to the corresponding refresh clock signal, the refresh signalsdefining a second refresh rate having a variable value that is afunction of the frequency of the refresh clock signal.
 45. The memorydevice of claim 44 wherein the refresh controller operates in thelow-power mode to vary the bias voltage as a function of the supplyvoltage to linearly increase the frequency of the refresh clock signalas the supply voltage decreases below the minimum value.
 46. The memorydevice of claim 44 wherein the refresh controller generates the biasvoltage to develop the refresh clock signal having a substantiallyconstant frequency in the normal mode and generates the bias voltage toincrease the frequency of the refresh clock signal as the supply voltagedecreases in the low-power mode.
 47. The refresh controller of claim 44wherein the refresh controller generates the bias voltage to develop therefresh clock signal having a substantially constant frequency in thenormal mode and outputs the supply voltage as the bias voltage todirectly control the frequency of the refresh clock signal as a functionof the value of the supply voltage in the low-power mode.
 48. The memorydevice of claim 44 wherein the refresh controller generates the biasvoltage to develop the refresh clock signal having a substantiallyconstant frequency in the normal mode, and operates in the low-powermode to generate the bias voltage to increase the frequency of therefresh clock signal as the supply voltage decreases when the supplyvoltage is less than or equal to the minimum value and greater than asecond minimum value, and further operates in the low-power mode tooutput the supply voltage as the bias voltage to directly control thefrequency of the refresh clock signal as a function of the value of thesupply voltage when the supply voltage is less than or equal to thesecond minimum supply value.
 49. The refresh controller of claim 44wherein the memory device is adapted to receive a bias voltageadjustment command that is generated when the refresh controller is tooperate in the low-power mode, the command being decoded and applied tothe refresh controller which, in response to the decoded command,generates the bias voltage to increase the frequency of the refreshclock signal as the supply voltage decreases.
 50. The memory device ofclaim 44 wherein the memory device comprises a dynamic random accessmemory.
 51. A computer system, comprising: a data input device; a dataoutput device; a processor coupled to the data input and output devices;and a memory device coupled to the processor, the memory deviceincluding, an address decoder coupled to an address bus; a read/writecircuit coupled to a data bus; a control circuit coupled to a controlbus; a memory-cell array coupled to the address decoder, controlcircuit, and read/write circuit, the memory-cell array storing dynamicdata; and a refresh controller coupled to the memory-cell array, controlcircuit, and address decoder for controlling a refresh rate of thedynamic data, the refresh controller being adapted to receive a supplyvoltage and including a refresh oscillator that generates a refreshclock signal having a frequency that is a function of a bias voltage,the refresh circuit operable in a normal mode when the supply voltage isgreater than a minimum value to apply a substantially constant biasvoltage to the refresh oscillator and to generate refresh signals inresponse to the corresponding refresh clock signal, the refresh signalsdefining a first refresh rate having a value determined by the frequencyof the refresh clock signal, and the refresh controller operable in alow-power mode when the supply voltage is less than or equal to theminimum value to vary the bias voltage as a function of the supplyvoltage and to generate refresh signals in response to the correspondingrefresh clock signal, the refresh signals defining a second refresh ratehaving a variable value that is a function of the frequency of therefresh clock signal.
 52. The computer system of claim 51 wherein therefresh controller operates in the low-power mode to vary the biasvoltage as a function of the supply voltage to linearly increase thefrequency of the refresh clock signal as the supply voltage decreasesbelow the minimum value.
 53. The computer system of claim 51 wherein therefresh controller generates the bias voltage to develop the refreshclock signal having a substantially constant frequency in the normalmode and generates the bias voltage to increase the frequency of therefresh clock signal as the supply voltage decreases in the low-powermode.
 54. The computer system of claim 51 wherein the refresh controllergenerates the bias voltage to develop the refresh clock signal having asubstantially constant frequency in the normal mode and outputs thesupply voltage as the bias voltage to directly control the frequency ofthe refresh clock signal as a function of the value of the supplyvoltage in the low-power mode.
 55. The computer system of claim 51wherein the refresh controller generates the bias voltage to develop therefresh clock signal having a substantially constant frequency in thenormal mode, and operates in the low-power mode to generate the biasvoltage to increase the frequency of the refresh clock signal as thesupply voltage decreases when the supply voltage is less than or equalto the minimum value and greater than a second minimum value, andfurther operates in the low-power mode to output the supply voltage asthe bias voltage to directly control the frequency of the refresh clocksignal as a function of the value of the supply voltage when the supplyvoltage is less than or equal to the second minimum supply value. 56.The computer system of claim 51 wherein the memory device is adapted toreceive a bias voltage adjustment command that is generated when therefresh controller is to operate in the low-power mode, the commandbeing decoded and applied to the refresh controller which, in responseto the decoded command, generates the bias voltage to increase thefrequency of the refresh clock signal as the supply voltage decreases.57. The computer system of claim 51 wherein the memory device comprisesa dynamic random access memory.